Magnesium alloy, magnesium alloy member and method for manufacturing same, and method for using magnesium alloy

ABSTRACT

A magnesium alloy of the present invention has the chemical composition that contains 0.02 mol % or more and less than 0.1 mol % of at least one element selected from yttrium, scandium, and lanthanoid rare earth elements, and magnesium and unavoidable impurities accounting for the remainder. A magnesium alloy member of the present invention is produced by hot plastic working of the magnesium alloy in a temperature range of 200° C. to 550° C., followed by an isothermal heat treatment performed in a temperature range of 300° C. to 600° C. The magnesium alloy is preferred for use in applications such as in automobiles, railcars, and aerospace flying objects. The magnesium alloy and the magnesium alloy member can overcome the yielding stress anisotropy problem, and are less vulnerable to the rising price of rare earth elements.

TECHNICAL FIELD

The present invention relates to magnesium alloys that contain traceamounts of yttrium, scandium, and lanthanoid rare earth elements, and tomagnesium alloy members that allow for easy plastic working in cold androom temperature ranges.

The present invention also relates to a method for manufacturing amagnesium alloy member that allows for easy cold working, and that ispreferred for use in applications such as in automobiles, railcars,aerospace flying objects, and housings of electronic devices.

BACKGROUND ART

These types of magnesium alloys are desired in applications where lightstructural members are needed. Examples of such structural memberapplications include automobiles, railcars, aerospace flying objects,and housings of electronic devices. However, use of magnesium alloys asstructural members has not been realized because of the considerabledifficulties involved in the plastic working in cold and roomtemperature ranges. Wrought magnesium alloys produced by processes suchas press-rolling and extrusion are also problematic in terms of yieldingstress anisotropy, because the basal plane {0001} crystal orientationbecomes in line with the working direction, and creates a largedifference between the tensile and compression yielding stresses. Asused herein, “cold temperature” means ordinary temperature or atemperature below the recrystallization temperature of the material. Thecold working temperatures of magnesium alloys are typically 200° C. orless.

PTL 1 and PTL 2 disclose wrought magnesium alloys that contain 0.1 to1.5 mol % of yttrium. These wrought magnesium alloys advantageouslyovercome the yielding stress anisotropy problem, and have excellent coldworkability. A problem, however, is that these materials containyttrium, and are vulnerable to the rising price of yttrium.

PTL 3 and PTL 4 disclose rolled magnesium alloys that contain 0.01 to0.5 mol % of yttrium. The advantage of these rolled magnesium alloys isthe low yttrium content. However, the basal plane is in line with thepress-roll direction (PTL 4, FIG. 1), and it is not difficult to imaginethat a large difference occurs between the tensile and compressionyielding stresses.

PTL 5 and PTL 6 disclose rolled magnesium alloys that contain only traceamounts of yttrium for easy workability. These rolled magnesium alloyscontain 6 to 16 mass % of lithium, and the β phase of the BCC(body-centered cubic lattice) structure is dispersed in the a phase ofthe HCP (hexagonal close-packed) structure to improve workability.However, the use of the active element lithium severely impairs thecorrosion resistance of the material, and poses a safety problem.

PTL 7 discloses a magnesium alloy in which quasicrystal grains aredispersed in the magnesium matrix in order to reduce yielding stressanisotropy. However, this magnesium alloy is a Mg—Zn—Re alloy,containing rare earth elements in a content of 0.2 to 1.5 mol %. Aproblem, then, is that the material is vulnerable to the rising price ofrare earths. There is indeed a need to reduce the rare earth content.

PTL 8 discloses a wrought magnesium alloy that contains 0.03 to 0.54 mol% of yttrium. This wrought magnesium alloy has an average magnesiumcrystal grain diameter of 1.5 μm or less, and a high concentration ofyttrium is segregated in the vicinity of the grain boundary to improvematerial strength. The solute element remains at high concentration inthe vicinity of the grain boundary when the size of matrix is fine andthe percentage volume of the grain boundary is high. However, the soluteelement exists in a solid solution state not in the vicinity of thecrystal grain boundary but inside the size of matrix in applicationswhere the crystals have coarse grain diameters (for example, 10 μm ormore). The material cannot have high strength in this case.

CITATION LIST Patent Literature

PTL 1: WO2010/010965

PTL 2: WO2008/117890

PTL 3: JP-A-2010-13725

PTL 4: JP-A-2008-214668

PTL 5: JP-A-2003-226929

PTL 6: JP-A-9-41066

PTL 7: JP-A-2010-222645

PTL 8: Japanese Patent No. 4840751

SUMMARY OF INVENTION Technical Problem

In magnesium alloys, strength and ductility are improved by making finegrains using press-rolling, extrusion, and other processes that applystrain, as with the case of other metallic materials. However, the basalplane {0001} becomes in line with the working direction, specifically abasal plane texture is formed during the hot working for reasonsattributed to the magnesium crystal structure. For example, the crystalorientation of the basal plane of press-rolled or extruded magnesiumaligns parallel to the press-roll or extrusion direction. This isproblematic in terms of yielding stress anisotropy, because thecompression yielding stress is only 50% to 60% of the tensile yieldingstress. There have been attempts to overcome this problem by dispersingquasicrystal grains (PTL 7) or producing alloys (PTLs 1 to 6). However,all of these techniques involve addition of 0.1 mol % or more of rareearth elements, and are vulnerable to the rising price of rare earths.

Solution to Problem

According to a first aspect of the present invention, there is provideda magnesium alloy that comprises 0.02 mol % or more and less than 0.1mol % of yttrium, scandium, or lanthanoid rare earth elements, and Mgand unavoidable impurities accounting for the remainder. The magnesiumalloy has a homogenous composition, and a homogenous crystal structurewith an average grain size of several micrometers and several tenmicrometers.

According to a second aspect of the present invention, there is provideda magnesium alloy member that is produced by hot plastic working of themagnesium alloy of the chemical composition of the first aspect of theinvention in a temperature range of 200° C. to 550° C., followed by anisothermal heat treatment performed in a temperature range of 300° C. to600° C. The isothermal heat treatment is a process by which a magnesiumalloy sample is placed in a maintained constant temperature bath,maintained for a predetermined time period, and slowly cooled in airoutside of the bath. The magnesium alloy member may be a wroughtmagnesium member such as a plate member, a rod member, and a pipemember.

A third aspect of the present invention is the magnesium alloy memberaccording to the second aspect in which the crystal structure of themember is an equiaxial grain structure with no texture. Equiaxial grainmeans a three-dimensionally isotropic crystal grain structure that doesnot stretch or flatten unidirectionally. Texture, or crystal texture asit is also called, refers to a distribution state of the crystal latticeorientation (crystal orientation) of each crystal grain present in apolycrystalline material such as metal. For example, solidifying acubical crystal metal forms a preferred orientation [100]. In the caseof magnesium, the basal plane {0001} tends to align in the strainapplying direction, as noted above.

A fourth aspect of the present invention is the magnesium alloy memberaccording to the second or third aspect in which the average grain sizeis 10 μm or more.

A fifth aspect of the present invention is the magnesium alloy membersaccording to any one of the second to fourth aspect in which acompressional nominal strain of 0.4 or more is applied by cold workingperformed in a temperature range of from room temperature (here andbelow, room temperature means 15° C. to 35° C.) to 150° C.

A sixth aspect of the present invention is the magnesium alloy membersaccording to any one of the second to fifth aspect in which the averagegrain size of the magnesium alloy after cold working performed in atemperature range of from room temperature to 150° C. is 80% or less ofthe initial average grain size (undeformed magnesium alloy).

A seventh aspect of the present invention is the magnesium alloy memberaccording to the third aspect in which the strength and hardness of themember after applying nominal strain by cold working performed in atemperature range of from room temperature to 150° C. are 15% greaterthan strength and hardness of the undeformed ones.

A first method of the present invention is a method for producing amagnesium alloy member, the method comprising: hot plastic working of amagnesium alloy that contains 0.02 mol % or more and less than 0.1 mol %of at least one element selected from yttrium, scandium, and lanthanoidrare earth elements, and Mg and unavoidable impurities accounting forthe remainder, the hot plastic working being performed in a temperaturerange of 200° C. to 550° C.; and an isothermal heat treatment of themagnesium alloy in a temperature range of 300° C. to 600° C. after thehot plastic working.

A second method of the present invention is a method for using amagnesium alloy that contains 0.02 mol % or more and less than 0.1 mol %of at least one element selected from yttrium, scandium, and lanthanoidrare earth elements, and Mg and unavoidable impurities accounting forthe remainder, the method comprising using the magnesium alloy as awrought magnesium member after hot plastic working performed in atemperature range of 200° C. to 550° C., and a subsequent isothermalheat treatment performed in a temperature range of 300° C. to 600° C.

Advantageous Effects of Invention

The present invention induces room temperature recrystallization (grainrefining) by controlling the dispersion state of one or more elementsselected from yttrium, scandium, and lanthanoid rare earth elements in amagnesium alloy. This makes it possible to develop excellentcompressional deformation characteristics. The magnesium alloy member ofthe present invention overcomes the yielding stress anisotropy problemwith its random crystal orientation distribution (after working), andhas the same yielding stress for the tensile and compressionaldeformation with the maintained high strength. Further, the magnesiumalloy member of the present invention does not break even under a largeapplied compressional strain in excess of 50%, and has excellentdeformability. Because of the considerably low yttrium, scandium, andlanthanoid rare earth element content, the magnesium alloy of thepresent invention is less vulnerable to the material price of yttrium,scandium, and lanthanoid rare earth elements as compared to conventionalrare earth-containing magnesium alloys.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a photographic representation showing the appearance of amaterial when the hot working temperature is in the appropriate range.

FIG. 2 represents the nominal stress-nominal strain curve obtained afterthe room-temperature tensile and compression testing of extrudedmaterial Mg-0.05Y and extruded and heat-treated material Mg-0.05Y.

FIG. 3 represents the nominal stress-nominal strain curve obtained afterthe room-temperature compression testing of extruded material Mg—Yalloy.

FIG. 4 represents the nominal stress-nominal strain curve obtained afterthe room-temperature compression testing of extruded and heat-treatedmaterial Mg—Y alloy.

FIG. 5 represents the nominal stress-nominal strain curve obtained afterthe room-temperature compression testing of cast and heat-treatedmaterial Mg-1 mol % Y.

FIG. 6 is a photographic representation of the observed scanningelectron micrograph/electron backscatter diffraction image of extrudedand heat-treated material Mg-0.03Y.

FIG. 7 shows a pole figure image of the region observed in FIG. 5, inwhich ED and TD are directions parallel to and perpendicular toextrusion direction, respectively.

FIG. 8 is a photographic representation of extruded and heat-treatedmaterial Mg-0.03Y as observed by scanning electron microscopy/electronbackscatter diffraction after applying 20% compressional strain.

FIG. 9 is a photographic representation of extruded and heat-treatedmaterial Mg-0.03Y as observed by scanning electron microscopy/electronbackscatter diffraction after applying 50% compressional strain.

FIG. 10 is a photographic representation showing the appearance of amaterial at a low hot working temperature.

DESCRIPTION OF EMBODIMENTS

The magnesium alloy of the present invention contains at least oneelement selected from yttrium, scandium, and lanthanoid rare earthelements, as mentioned above. A magnesium alloy containing yttrium, andan alloy member of such an alloy will be described below as embodimentsof the magnesium alloy and the magnesium alloy member of the presentinvention.

For the magnesium alloy member of the present invention to exhibiteffect, hot plastic working (hereinafter, also referred to as “hotworking”) of a magnesium alloy is required to segregate yttrium to agrain boundary, and the yttrium needs to be diffused in the graininterior by isothermal heat treatment. The procedures are as follows.

For the magnesium alloy member of the present invention to exhibiteffect, the magnesium alloy contains yttrium in 0.02 mol % or more andless than 0.1 mol %, and magnesium and unavoidable impurities accountingfor the reminder. The yttrium content is preferably 0.025 mol % or moreand less than 0.1 mol %, more preferably 0.025 mol % or more and lessthan 0.05 mol %. When the yttrium content is 0.02 mol %, the yttriumexists at 19.5×10⁻¹⁰ m radius intervals. This value corresponds to themagnitude about three times the Burgers vector of magnesium, andrepresents a value that limits the interactions of lattice defects suchas dislocations in terms of atomic binding theory. The grain size thatallows the yttrium to homogenously segregate to a grain boundary by thehot working becomes coarser as the yttrium content decreases. It is,however, difficult to obtain the effect because the estimated averagegrain size after the hot working is 10 μm or more. Here, the Burgersvector represents the distorted direction of atoms around a dislocationline introduced as a crystallographic linear crystal defect. In edgedislocations, the dislocation line and the Burgers vector areperpendicular to each other, whereas these are parallel to each other inscrew dislocations.

The hot plastic working temperature is preferably 200° C. to 550° C.,more preferably 250° C. to 350° C. When the working temperature is below200° C., the low working temperature makes dynamic recrystallizationless likely to occur. FIG. 1 is a photographic representation showingthe appearance of a material when the hot working temperature is in theappropriate range. FIG. 10 is a photographic representation showing theexterior of a material at a low hot working temperature. By comparingFIG. 1 and FIG. 10, it can be seen that an appropriate magnesium alloymember can be produced by setting the hot working temperature in theappropriate temperature range. Above 550° C., the high workingtemperature makes it difficult to produce an average grain size of 10 μmor less. There is also a potential problem in mold lifetime such as inextrusion. The hot working is typically extrusion, forging,press-rolling, or drawing. However, any plastic working may be used, aslong as strain can be applied. The equivalent plastic strain duringstrain application is 1.5 or more, preferably 2.0 or more. When theequivalent plastic strain is less than 1.5, a sufficient strain cannotbe applied, and a mixed structure of coarse grains and fine grainsappears, making it difficult to homogenously segregate the yttrium inthe vicinity of the grain boundary. With the isothermal heat treatmentof a cast material alone without the hot working, the yttrium does nothomogenously diffuse and disperse in the grain interior, and fractureoccurs at a nominal strain of about 0.3 as shown in FIG. 5. That is, theeffects of the present invention cannot be obtained.

The temperature of the isothermal heat treatment is preferably equal toor greater than the hot working temperature, so that the yttriumsegregated at the grain boundary can diffuse in the grain interior.Specifically, temperature of the isothermal heat treatment is preferably300° C. to 600° C., more preferably 350° C. to 450° C. A heat treatmenttemperature above 600° C. may cause the material to burn during the heattreatment. The retention time, which varies with the heat treatmenttemperature, is preferably 3 minutes to 24 hours. A retention timelonger than 24 hours has the possibility of causing abnormal graingrowth during the heat treatment.

The magnesium alloy member having an equiaxial grain structure with notexture can be obtained in this manner. It is also possible to obtain amagnesium alloy member having an average grain size of 10 μm or more,for example 30 μm to 50 μm.

The magnesium alloy member may be subjected to cold plastic working.(hereinafter, also referred to as “cold working”) in a temperature rangeof from room temperature to 150° C. For example, a compressional nominalstrain of 0.4 or more can be applied. The upper limit is 1.5. The coldworking refines the crystal grains of the magnesium alloy member. Forexample, the size of matrix can be refined to 80% or less of the averagegrain size of the magnesium alloy member after the cold working. Thelower limit is 5%, though it is not particularly limited.

Refining of the crystal grains by cold working can increase the hardnessand strength of the magnesium alloy member. For example, the hardness ofthe magnesium alloy member can be increased 15% or more after the coldworking. The strength of the magnesium alloy member also can beincreased 15% or more after the cold working.

As described above, in the present embodiment, the dispersion state ofyttrium is controlled by hot working the magnesium alloy to segregateyttrium at a grain boundary, and performing an isothermal heat treatmentto diffuse the yttrium in the grain interior. The subsequent coldworking refines the grains, and improves the hardness and strength ofthe magnesium alloy member.

Controlling the dispersion state of yttrium induces room temperaturerecrystallization (grain refining), and excellent compressionaldeformation characteristics can be developed.

The foregoing embodiments described the yttrium-containing magnesiumalloy, and the alloy member of such a magnesium alloy. However, the,present invention is not limited to these embodiments. The presentinvention also encompasses a magnesium alloy and an alloy member inwhich some of or all of the yttrium are substituted with scandium orlanthanoid rare earth elements such as lanthanum and cerium, or withscandium and lanthanoid rare earth elements. Scandium, and lanthanoidelements such as lanthanum and cerium belong to the same group asyttrium, and are located above and below yttrium in the periodic table.These elements thus have many similarities in chemical and physicalproperties, and the present invention can sufficiently exhibit effecteven when some of or all of the yttrium are substituted with theseelements. The desired effect of the present invention can be moreeffectively obtained with the yttrium-containing magnesium alloy, andthe alloy member of such a magnesium alloy.

EXAMPLES

Yttrium (Y) and pure magnesium (Mg; purity 99.95%) were completelymelted in an argon atmosphere, and cast into an iron mold to fabricatefive types of Mg—Y alloy cast materials with the target Y contents of0.01 mol %, 0.02 mol %, 0.03 mol %, 0.04 mol %, and 0.05 mol %. Thetarget Y contents of 0.03 mol %, 0.04 mol %, and 0.05 mol % fall withinthe range of the present invention (Examples). The target Y contents of0.01 mol % and 0.02 mol % fall outside of the range of the presentinvention (Comparative Examples). The Y content, and the concentrationsof other composition elements were evaluated by ICP atomic emissionspectrometry after a 2-hour solution treatment of the cast material at500° C. The results of the composition analysis are presented inTable 1. The five alloys were produced by using the following proceduresunder the following conditions.

TABLE 1 Y Fe Si Mn Cu Mg—0.05Y 0.16 (=0.044) 0.002 0.002 0.003 <0.001Mg—0.04Y 0.13 (=0.036) 0.002 0.002 0.004 <0.001 Mg—0.03Y 0.09 (=0.025)0.002 0.002 0.004 <0.001 Mg—0.02Y 0.06 (=0.016) 0.002 0.002 0.003 <0.001Mg—0.01Y 0.02 (=0.005) 0.002 0.002 0.003 <0.001 Figures in parenthesesare mol %. Other figures are mass %.

The cast material was maintained in a furnace at a temperature of 500°C. for 2 hours, and then water cooled as a solution treatment. Theproduct was then machined to produce a columnar extrusion billetmeasuring 40 mm in diameter and 70 mm in height. The same unit billetwas maintained for 30 minutes in a container maintained at the extrusiontemperature shown in Table 2, and subjected to a hot strain applyingprocess, which was performed by extruding the material at an extrusionratio of 25:1. The resulting product will be called “extruded material.”The average equivalent plastic strain was 3.7 as determined from thepercentage reduction of a cross section. The extruded material wasisothermally maintained in a 400° C. furnace for 15 minutes, and allowedto cool in air to prepare a sample. This product will be called“extruded and heat-treated material.”

TABLE 2 Y concentration, Extrusion Heat treatment Heat treatment grainsize, Fracture mol % temperature, degrees temperature, degrees time, minμm Tys, MPa Cys, MPa strain Mg—0.05Y 0.044 306 — — 5 278 140 0.15 0.044306 400 15 32 91 65 >0.50 Mg—0.04Y 0.036 302 — — 5 252 148 0.15 0.036302 400 15 38 91 65 >0.50 Mg—0.03Y 0.025 315 — — 5 207 134 0.14 0.025315 400 15 40 85 65 >0.50 Mg—0.02Y 0.016 304 — — 75 94 68 0.17 0.016 304400 15 94 84 34 0.31 0.016 212 — — 5 116 103 0.38 0.016 212 400 15 44100 47 0.36 Mg—0.01Y 0.005 317 — — 75 92 47 0.27 0.005 317 400 15 >10032 0.32 0.005 238 — — 30 97 65 0.35 0.005 238 400 15 65 80 34 0.33 Tys:Tensile yielding stress, Cys: Compressional yielding stress

Mg—Y alloy samples collected from the extruded materials and theextruded and heat-treated materials were subjected to a room-temperaturetensile and compression test at a strain rate of 1×10⁻³ s⁻¹. All testsamples were collected in a direction parallel to the extrusiondirection. FIGS. 2 to 4 represent the nominal stress-nominal straincurves obtained after the room-temperature tensile and compression test.It can be seen that fracture occurs in the extruded materials in thenominal strain range of 0.2 to 0.3, irrespective of the amounts ofyttrium added. Here, “fracture” is defined as at least 20% reduction instress, and denoted as BK in the figures. A fracture occurred in theextruded and heat-treated materials Mg-0.01Y and Mg-0.02Y in the nominalstrain range of 0.2 to 0.3 as in the extruded materials. However, nofracture occurred in the extruded and heat-treated materials Mg-0.03Y,Mg-0.04Y, and Mg-0.05Y even under the applied nominal strain of 0.5.These results suggest that the extruded and heat-treated materialsMg-0.03Y, Mg-0.04Y, and Mg-0.05Y are highly suited for cold working. AsComparative Example, a Mg-1 mol % Y alloy was fabricated by casting, andsubjected to a room-temperature compression test after a solutiontreatment, without performing hot working. The result is shown in FIG.5. It can be seen that fracture occurs at a nominal strain as low asabout 0.3, despite the high yttrium content. The post-casting hot strainapplying process can thus be said as essential for the present inventionto exhibit effect.

FIG. 6 shows an example of the observed scanning electronmicrograph/electron backscatter diffraction image of the extruded andheat-treated material Mg-0.03Y. The symbols. ED and TD representdirections parallel and perpendicular to the extrusion direction,respectively. It can be seen that the material does not tensile in theextrusion direction: ED, and has an equiaxial structure. The averagediameter of grains with 15° or greater misorientation was 40 μm. FIG. 7shows a pole figure image of the region observed in FIG. 6. Each pointcorresponds to the crystal orientation of the measured crystal grain. Itcan be seen that the material has a random texture without accumulationof basal plane in the specific direction (extrusion direction).

FIG. 8 shows an example of the microstructure of the extruded andheat-treated material Mg-0.03Y observed by scanning electronmicroscopy/electron backscatter diffraction after applying 20%compressional nominal strain (=0.20). In contrast to the undeformedmaterial of FIG. 6, refining of the grains can be observed. The averagediameter of the grains with 15° or greater misorientation was 30 μm, 75%of the initial grain diameter before the room temperaturerecrystallization. In the figure, the symbol “LG” represents a low-anglegrain boundary with less than 15° misorientation. This is considered tobe largely due to the room temperature recrystallization forming alow-angle boundary of about 5° in the grains. FIG. 9 shows an example ofthe microstructure of the extruded and heat-treated material Mg-0.03Yobserved by scanning electron microscopy/electron backscatterdiffraction after applying 50% compressional nominal strain (=0.50). Incontrast to the undeformed material of FIG. 6, refining of the grainscan be observed. The average diameter of the crystal grains with 15° orgreater misorientation was 11 μm, 25% of the initial grain diameter.

Hardness measurement was performed for the extruded and heat-treatedmaterial Mg-0.03Y (undeformed sample) and the sample to which 50%compressional nominal strain (=0.50) was applied. Hardness was 30.5 Hvfor the undeformed sample, and 36.5 Hv for the 50% deformed sample. Theimproved hardness over the undeformed material is attributed to thefiner grain size imparted after the room temperature working. It can beseen from these results that the material of the present inventionimproves hardness and strength after the room-temperature plasticworking.

COMPARATIVE EXAMPLE

When the working temperature is below 200° C., the low workingtemperature makes dynamic recrystallization less likely to occur. FIG.10 is a photographic representation showing the appearance of thematerial of Comparative Example, representing the situation wheredynamic recrystallization is limited by low working temperature. Thelimited dynamic recrystallization makes the material of ComparativeExample less usable in producing a good material.

INDUSTRIAL APPLICABILITY

The present invention induces room temperature recrystallization (grainrefining), and develops excellent compressional deformationcharacteristics by controlling the dispersion state of one or moreelements selected from yttrium, scandium, and lanthanoid rare earthelements in a magnesium alloy. The magnesium alloy member of the presentinvention has a random crystal orientation distribution (after working),and the same yielding stress for the tensile and compressionaldeformation with the maintained high strength. The magnesium alloymember of the present invention can thus be used as a wrought magnesiummember such as a plate member, a rod member, and a pipe member. In athree-dimensional structure using such a wrought magnesium member, anyexternal force acting on the structure deforms the magnesium alloymember near isotropically, and the strengths against the locally actingtensile and compressional loads become essentially the same. Further,the magnesium alloy member of the present invention does not break evenunder a large applied compressional strain in excess of 50%, and hasexcellent deformability. The magnesium alloy member of the presentinvention can thus be used as a structural member or a shock absorbingmaterial in applications such as automobiles, railcars, aerospace flyingobjects, and portable electronic devices.

1. A magnesium alloy comprising 0.02 mol % or more and less than 0.1 mol% of at least one element selected from yttrium, scandium, andlanthanoid rare earth elements, and Mg and unavoidable impuritiesaccounting for the remainder.
 2. A magnesium alloy member comprising themagnesium alloy of the chemical composition of claim 1, wherein thecrystal structure of the member is an equiaxial grain structure with notexture.
 3. (canceled)
 4. The magnesium alloy member according to claim2, wherein the average grain size is 10 μm or more.
 5. The magnesiumalloy member according to claim 2, wherein a compressional nominalstrain of 0.4 or more is applied by cold working performed in atemperature range of from room temperature to 150° C.
 6. The magnesiumalloy member according to claim 2, wherein the average grain size of themagnesium alloy after cold working performed in a temperature range offrom room temperature to 150° C. is 80% or less of the average grainsize of an unworked magnesium alloy.
 7. The magnesium alloy memberaccording to claim 2, wherein the strength and hardness of the memberafter applying nominal strain by cold working performed in a temperaturerange of from room temperature to 150° C. are 15% greater than strengthand hardness of the undeformed ones.
 8. A method for producing amagnesium alloy member, the method comprising: hot plastic working of amagnesium alloy that contains 0.02 mol % or more and less than 0.1 mol %of at least one element selected from yttrium, scandium, and lanthanoidrare earth elements, and Mg and unavoidable impurities accounting forthe remainder, the hot plastic working being performed in a temperaturerange of 200° C. to 550° C.; and an isothermal heat treatment of themagnesium alloy in a temperature range of 300° C. to 600° C. after thehot plastic working.
 9. A method for using a magnesium alloy thatcontains 0.02 mol % or more and less than 0.1 mol % of at least oneelement selected from yttrium, scandium, and lanthanoid rare earthelements, and Mg and unavoidable impurities accounting for theremainder, the method using the magnesium alloy as a wrought magnesiummember after hot plastic working performed in a temperature range of200° C. to 550° C., and a subsequent isothermal heat treatment performedin a temperature range of 300° C. to 600° C.